In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The wireless terminals can be mobile stations or user equipment units (UE) such as mobile telephones (“cellular” telephones) and laptops with wireless capability, e.g., mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks is also called “NodeB”, “B node”, or (in LTE) eNodeB. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of radio access networks, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). An entity known as the Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
Specifications for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) are ongoing within the 3rd Generation Partnership Project (3GPP). Another name used for E-UTRAN is the Long Term Evolution (LTE) Radio Access Network (RAN). Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are performed by the radio base stations nodes. As such, the radio access network (RAN) of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes. The evolved UTRAN (E-UTRAN) comprises evolved base station nodes, e.g., evolved NodeBs or eNBs, providing evolved UTRA user-plane and control-plane protocol terminations toward the wireless terminal.
As those skilled in the art appreciate, in UTRAN- or WCDMA-based technology a common frequency band allows simultaneous communication between a user equipment unit (UE) and plural base stations. Therefore a user equipment unit (UE) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support a connection to a user equipment unit (UE) at the same time the origination cell continues to service the connection. Since the user equipment unit (UE) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
In wireless communication networks, the mobility management is responsible for handover between the cells and for cell reselection. The handover has a strong impact on the quality of service perceived by the user, because it may generate delay to the packet transfer times as well as call or connection drop. Thus, good performance of the mobility management mechanisms is crucial in success of new radio network technologies.
When evaluating the performance of handover, a number of performance criteria can be used. A commonly used one is the percentage of failed handovers. However, the handover failures cannot be used in eliciting effective preventative actions to improve the performance, because a failure implies already dropped connection and quality degradation.
The triggers that define when the handover decision is taken influence the performance of the procedure significantly. As an example, the parameters in the case of handover in LTE network comprise the hysteresis in received signal strength and the time hysteresis before making the handover decision. In the case of cell reselection in idle mode of LTE, the parameters used in mobility handling are Qhyst and Treselection and they account for the hysteresis in the received signal strength and time hysteresis respectively.
Discontinuous Reception (DRX) mechanism is defined for LTE in both RRC_IDLE and RRC_CONNECTED modes by 3GPP and is a mechanism to save battery resources of the terminal. The terminal only needs to be active a fraction of time in the beginning of predefined cycles for receiving DL/UL assignment information. DRX in connected mode is typically used in case when a small amount of data is transmitted to the terminal (VoIP, “Pings” etc) or when there are longer inactivity periods between downloads in web access. With DRX, the UE can turn on and off reception of L1/L2 control in RRC_CONNECTED state. None of the mobility related measurements is done when the terminal is in DRX. Thus, the length of DRX cycle has also impact on the handover performance.
Some proposals for improving handover performance exist already, a few of which are listed below:                Adapting the DRX cycle when an event is detected (WO 2008/082347 by Kazmi M et al. and US 2008/0160918 A1 by Kyeong-In Jeong, Van Lieshout, et al.).        Adapting DRX Cycle in function of absolute values of received signal strength (e.g. If RSS below a threshold), or RSS difference between serving and target cell (US 2008/0160918 A1 by Kyeong-In Jeong, Van Lieshout, et al.).        Adapting of DRX cycle in function of the observed traffic statistics and HARQ feedback (20080167089 A1 89)        
Exchange of DRX information during HO between serving and target cell (WO 2008086649 A1). Resuming the use of DRX from target cell after the serving cell transmits a HO command (US 20080090573 A1).                Adapting the layer 3 criteria so as to adjust timers controlling RLF recovery, on the basis of L3 criteria, which are available, scaled by DRX cycles        The UE estimates a suggested DRX cycle and notifies the network. As a by-product of the suggested DRX cycle adaptations, it is already suggested that the layer 3 filter coefficient k is adapted in function of the DRX cycle        
Mainly, handover is an important radio resource control procedure whose performance has a strong impact on the quality of service perceived by the user, due to the call or connection drops the handover can generate.
A number of handover performance criteria are used. A commonly used one is the percentage of failed handovers. However, the use of handover failures cannot lead to effective preventative actions, since failures imply already dropped connections.
Operators would like to maintain the target grade of service (lower blocking and dropping) and good quality of service. Therefore a number of criteria set by the operators, need to be fulfilled. In real network the desired grade of service can be achieved provided UE is connected or camped on to the best cell most of the time. This ensures high reception level or SINR, which in turn improves user bit rate and improvement in overall performance. But due to varying radio conditions and UE mobility it is though challenging though important that connection to the best cell is retained.